The function of a rotary yarn guide in a spinning machine is to guide the yarn along the prolongation of the axis of the spindle and to impart rotation to it in order to give it a supplementary twist. This twist ensures that the yarn will have increased strength, which reduces breakages during its production.
A rotary yarn guide comprises a guide member through which the yarn passes. This guide member is composed, in known yarn guides, of a very hard material, for example a ceramic material, in order to avoid wear or polishing of the wall of the hole through which the yarn passes. The guide member is arranged to rotate about its axis at a speed of rotation higher than that of the yarn. To this end, a ball bearing is fitted between the guide member and a support therefor and a permanent magnet is fixed to the guide member and is coupled with another magnet carried by the spindle.
The external shoulders and projections provided on the guide members of the known yarn guides make them difficult to manufacture. Grinding, which would enable very small tolerances to be achieved, is in practice generally excluded for reasons of the cost of manufacture. It was necessary therefore to accept wide tolerances in the outside diameters of the guide member and to fix other parts of the yarn guide on the said guide member, with the aid of the intermediate members such as cages, by gluing. Such fixing by gluing, however, increases the assembly time, reduces the concentricity of the component parts and makes the replacement of any one such component part, and especially the ball bearing, practically impossible.